The present invention relates to a device and method of monitoring elapsed time in a controlled environment. The present invention has particular applicability in monitoring the elapsed time of an object in a controlled environment, such as an object in a semiconductor fabrication clean room.
During conventional semiconductor manufacturing, a batch or lot of semiconductor wafers undergo a multitude of processing steps including cleaning, coating, etching and testing. Carriers such as cassettes, boxes or boats are used to store, transport and transfer semiconductor wafers or a batch thereof from one processing step to another. Carriers are designed for the purpose of supporting the manufacturing and processing machinery which may either process the wafer while within the carrier or may permit removal for a particular operation and return of the wafers after a process operation.
Extreme demands placed on device features requires that the environment for housing the device fabrication process be stringently controlled for temperature, humidity, and environmental contaminants such as air borne chemicals or particles which may transferred to and damage individual or a batch of wafers. The potential sources of contamination include personnel, processing equipment, treatment chemicals, moving air and handling containers. The contaminants and particles are called defects which reduce semiconductor device yields.
The carriers themselves are also subject to contaminants and the fabrication process and are periodically cleaned to remove particles and/or chemical contaminants Because of the close and constant proximity of the wafers to the carriers, it is imperative that the carriers are maintained free of damaging contaminants. Due to the nature of these contaminants however, mere visual inspection of the carriers is inadequate and proper maintenance requires estimating the length of time that a carrier has been circulating within the processing operation.
Because of contamination concerns and the necessity of minimizing process variation, semiconductor wafers are tracked through many process steps to maintain a high level of control over the complete manufacturing process. Conventionally, semiconductor wafers have been tracked by a variety of systems including, for example: manual entries, such as by generating manual records; semi-automated systems, such as by a keyboard or bar code entry at various processing stations; or by automated systems through the use of elaborate radio frequency generators and transponder tags. Although tracking systems have been developed for the overall semiconductor device fabrication and for the tracking of a batch of wafers, there exists a need for a convenient method of tracking the time intervals among steps of critical operations within the overall process.
Carriers play a critical role in the manufacturing process, although the length of exposure time during the fabrication process and consequently their probability of contamination have not been the subject of adequate tracking, if at all. The exposure time is particularly difficult to estimate, since some operations require that wafers from one carrier be transferred to another carrier. Thus the carriers, as well as other equipment used during the manufacture of semiconductor devices, are independent of wafer tracking systems.
A variety of devices have been used to indicate exposure of an article to a certain level of contamination. Typically, the focus has been placed on identifying the amount or type of an alien gas or the relative humidity contained in an enclosed environment. Generally, these devices employ an indicating detector to signal that the surrounding environment, has reached unsuitable condition due to the presence or specified the contaminants. In addition, these devices are used to absorb the offending contaminant such as the absorption of moisture with an indicating desiccant. There are five main types of desiccants currently on the market: molecular sieves, activated alumnia, activated carbon, silica gel and clay. Although indicating devices have been known for signaling and absorbing specified contaminants in an enclosed environment, there has been no recognition that in a controlled environment, where the temperature, humidity and gas composition remain constant, a contamination indicating device may be employed as a timer.
Several simple and inexpensive contamination recording devices are known. For example, U.S. Pat. Nos. 5,224,373 and 4,098,120 describe the use of indicators for monitoring the relative humidity level of an enclosed container. The humidity indicators disclosed in these patents employ salts that change color upon exposure to certain levels of relative humidity, thereby permitting a visual indication of the level of humidity in the atmosphere of containers. Other humidity indicators have also been developed which employ a composition including an acidified aldehyde as, for example, an bacidified vanillin. U.S. Pat. No. 5,112,768 discloses methods and a device for sensing moisture by exposing a vanillin coated device to a test atmosphere. Humidity in the atmosphere chemically reacts with the coating to change its color, thereby indicating a level of moisture in the atmosphere.
Other examples of devices useful for detecting environmental conditions include the detection of an alien gas by a gas sensor. U.S. Pat. No. 5,439,648, for example, discloses a gas indicator to detect for the amount of carbon dioxide within the atmosphere of a food package thereby indicating the condition of the food product contain therein. The indicator provides an immediate warning by changing colors if the atmosphere inside the package changes to an unsuitable composition. The indicator is formed by utilizing a carbon dioxide sensitive ink printed as a set of strips on the sensor. Individual strips will change color depending on the amount of carbon dioxide in the package.
Although methods for detecting gases and humidity within an environment have been known as environmental condition monitors, the use of indicators in a controlled environment, such as a semiconductor clean room, is considered superfluous in that the environment does not greatly vary in composition or temperature. The use of indicators in a controlled environment as timers, however, has not been recognized.
Due to the precision and control required in semiconductor manufacturing, a need exists for improved monitoring of time interval between critical manufacturing operations. There is also a need to conveniently track the exposure time that a contaminant-sensitive object has been subjected to a controlled environment to accurately determine a cleaning or decontamination time for the object.
An advantage of the present invention is a device for measuring an elapsed time in a controlled environment. The device offers the advantage of conveniently and inexpensively tracking the length of time that an object has been exposed to the controlled environment.
Another advantage of the present invention is a method for monitoring elapsed time in a controlled environment. The method provides a simple means to monitor the length of time between processing steps.
Additional advantages and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a timer comprising a substrate and a substance-sensitive material on the substrate. It is advantageous for the substance-sensitive material to have a color which varies in accordance with exposure to a substance in the controlled environment thereby permitting convenient visual indicia of elapsed exposure time. A permeable transparent layer substantially covering the substance-sensitive material is further provided to prevent contamination of the controlled environment by the timer.
Another aspect of the present invention is a method of measuring elapsed time in a controlled environment. The method comprises: exposing a timer having a first readable exposure level to the controlled environment; and monitoring the timer for a second readable exposure level.
Additional advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following detailed description, wherein the embodiments of the present invention are described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modification s in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.